1. Field of the Invention
The present invention relates to a paper transfer apparatus incorporated in an electrophotographic printer used for printing selected images on elongated paper.
2. Description of the Related Art
To describe the overall arrangements of a typical paper transfer apparatus, reference is first made to FIG. 1 of the accompanying drawings. As shown in FIG. 1, a paper transfer apparatus may include a paper hopper 11, a pair of paper tractors 2 (only one is shown), a photosensitive drum 12, an image transferring unit 13, a discharge or scuff roller 16, a pinch roller 17 and a paper stacker 18.
The paper hopper 11 accommodates elongated paper P in an alternately folded manner. The elongated paper is formed with a series of feed holes arranged along each longitudinal edge of the paper. After being pulled out from the paper hopper 11, the paper P is brought into engagement with the tractors 2. The two tractors 2 are spaced from each other by a predetermined distance in a widthwise direction of the elongated paper P. Each of the tractors 2 includes an endless belt 21 formed with a plurality of projections 22. In operation, these projections come into engagement with the feed holes of the paper P. Thus, when the endless belts 21 are rotated, the paper P is caused to advance along a transfer path.
The photosensitive drum 12 has a cylindrical side surface on which electrostatic latent images are produced by an electrostatic latent image forming unit 14. Thus produced latent images are made visible or developed by a developing unit 15. Specifically, the latent images on the photosensitive drum 12 are changed to toner images by the developing unit. Then, the toner images are transferred onto the paper P by the transferring unit 13. Thereafter, the transferred toner images are thermally fixed to the paper by an image fixing unit 19. Then, the paper P, held between the discharge roller 16 and the pinch roller 17, is moved further along the transfer path. Finally, the paper P is received in the stacker 18 in an alternatively folded manner.
For performing high-quality printing, the transferring operation for the elongated paper needs to be synchronized to the rotation of the photosensitive drum 12. For attaining such synchronous operation, according to the arrangements shown in FIG. 1, use is made of the tractors 2 and the discharge roller 16 associated with the pinch roller 17. The tractors 2, arranged at an upstream point of the transfer path, are capable of moving the elongated paper at a rate equal to the peripheral speed of the photosensitive drum 12 rotated. The discharge roller 16 together with the pinch roller 17, arranged at a downstream point of the transfer path, is capable of applying a pulling force to the paper P in the transferring direction. For generating appropriate tension in the elongated paper P located between the tractors 2 and the discharge roller 16, the discharge roller 16 is caused to rotate at a rate greater than the predetermined transfer speed of the paper P (i.e., the peripheral speed of the photosensitive drum).
A conventional electrophotographic printer incorporating a paper transfer apparatus having the above arrangements has been found disadvantageous in the following points.
Specifically, as the thickness of the elongated paper used for a conventional printer is reduced, the feed holes of the paper tend to be more easily broken when the paper goes through the tractors 2. Even if such a breakage does not occur, the paper may be prematurely jerked out of engagement with the tractors 2. These anomalies will lead to defective printing results such as images printed at unintended locations on the paper.
The inventor of the present invention has found out that the above anomalies are caused by the following phenomena. First, as the thickness of the paper becomes smaller, the paper will be more strongly attached to the photosensitive drum 12 due to the corona discharge occurring at the time of image transfer. Second, as the rotational speed of the photosensitive drum 12 is increased for enabling more effective printing, it becomes harder for the elongated paper to be detached from the photosensitive drum 12 after the developed images are transferred onto the paper. This is because sufficient air cannot flow in between the photosensitive drum 12 and the paper as the rotational speed of the photosensitive drum is increased. As a result, even after the image transfer is finished, the paper is not detached immediately but will remain to be attached to the photosensitive drum for a while, as shown in FIG. 8. This means that the elongated paper is forced to move downstream to a greater extent than is originally expected. In such an instance, the feed holes of the paper may be broken or the paper may prematurely come out of engagement with the tractors 2, as stated above.